Current automotive catalysts for exhaust treatment of gasoline-powered vehicles include three way catalysts (TWCs) or four way catalysts (FWCs™). Such catalysts utilize palladium (Pd) and rhodium (Rh) as active species for conversion of hydrocarbons, CO, and NOx pollutants into harmless CO2, N2 and H2O. Pd is an active component for oxidation of hydrocarbons and CO into CO2, and Rh is the most efficient component for conversion of NOx into N2. Accordingly, both Pd and Rh are generally required for simultaneous high conversion of these three pollutants into harmless products. One of the main challenges in TWC design is how to most effectively use Rh. H. S. Gandhi et al., Journal of Catalysis, 2003, 216, 433-442, p. 435 provides a comprehensive overview of scenarios of Rh deactivation in TWCs. Rh deactivation mechanisms include rhodium aluminate formation, dissolution of Rh into alumina support material, encapsulation of the Rh by alumina, and spreading and interaction of Rh oxide over the alumina support surface. When Rh is utilized in the presence of Pd under oxidizing conditions up to temperatures of about 1000 K, Pd—Rh alloys can form and the Pd can form PdO that covers the surface of the Pd—Rh alloys, which can strongly suppress NOx conversion. To avoid undesirable formation of Pd—Rh alloys, current Pd/Rh three way catalyst formulations often use “fixation” of Pd and Rh on separate support phases (H. S. Gandhi et al., Journal of Catalysis, 2003, 216, 433-442, p. 437). However, even if Pd and Rh are loaded on separate support phases, formation of Pd—Rh alloy particles with an average size of >100 nm can still be observed using transmission electron microscopy (TEM) characterization. G. W. Graham et al., Catalysis Letters, 2002, 81, 1-7 (showing such particles after redox aging of a bimetallic Pd- and Rh-containing catalyst at 1050° C. for 12 h and noting that the surface of the Pd—Rh alloy particles was enriched with Pd, which is thought to be undesirable).
M. Rassoul et al., Journal of Catalysis, 2001, 203, 232-241 describe a bimetallic Pd—Rh/Al2O3 catalyst prepared by co-impregnation or stepwise impregnation of alumina from solutions of RhCl3 and H2PdCl4. Furthermore Rassoul et al. teach that Rh2O3 and PdO particles on the surface of catalysts obtained by the co-impregnation technique behave like respective monometallic catalysts. On the other hand, when the catalyst was prepared by stepwise impregnation, some Rh2O3 and PdO oxide particles were in strong interaction. While the addition of Rh by a stepwise impregnation technique improved the thermal stability of PdO, the majority of the Rh was lost in the bulk of the support. Rassoul et al. does not provide any indication of formation of bimetallic palladium-rhodium nanoparticles. Another example of a Pd—Rh/Al2O3 catalyst prepared by co-impregnation of Pd and Rh solutions on high surface area alumina support is described by Y. Reneme et al., Applied Catalysis B: Environmental, 2014, 160-161, 390-399.
Nunan et al. in SAE Meeting Paper (ISSN 0148-7191) N.950258 (1995) describe the impact of Pt—Rh and Pd—Rh interactions on the performance of bimetal catalysts. Under laboratory aging conditions, Nunan et al. conclude that nonalloyed Pt—Rh and Pd—Rh catalysts were “dominated by the Rh activity, whereas the alloyed Pd—Rh catalyst's performance was similar to that of the single-metal Pd catalyst. Pt—Rh or Pd—Rh alloying strongly impaired high-temperature hydrocarbon conversion or NO(sub)x conversion, respectively.” Nunan et al. determined that superior performance is achieved when Pd and Rh catalysts are prepared so as to prevent alloying.
Goto et al. disclose that formation of a Pd—Rh core-shell structured alloy with Rh in its core has a large negative impact on NOx performance. SAE Technical Paper 2014-01-1503, 2014, doi:10.4271/2014-01-1503. Examples in Goto et al. discuss the formation of homogeneously dispersed Pd and Rh particles on La/Al2O3 support material using a proprietary fixation method and partial formation of Pd—Rh alloyed nanoparticles was observed after aging at 1000° C. in size ranges of 20 to 50 nm. The paper states that there are benefits of a strategically-designed coexistence of Pd and Rh. According to the paper, an optimal Pd/Rh ratio is 1.2 in a top layer, which achieved better light-off activity compared to a Pd/Rh ratio of 2.4 where no Pd was in the Rh layer.
A. A. Vedyagin et al. describe a Pd—Rh alloy catalyst prepared by impregnation of alumina support with dual complex salt [Pd(NH3)4]3[Rh(NO2)6]2 in Topics in Catalysis, 2013, 56, 1008-104 and in Catalysis Today, 2014, 238, 80-86.
J. R. Renzas et al. disclose an examination of bimetallic 15 nm Pd-core Rh-shell Rh1-xPdx nanoparticles deposited on Si wafers for CO oxidation by O2 and CO oxidation by NO, respectively in Phys. Chem. Chem. Phys., 2011, 13, 2556-2562 and in Catalysis Letters 2011, 141, 235-241.
Tao et al. describe that heterogeneous catalysts containing bimetallic nanoparticles may undergo segregation of the metals, driven by oxidizing and reducing environments. Science, 2008, 322, 932-934
In U.S. Patent Appin. No. 2012/0263633 metal oxide support materials containing nanoscaled iron-platinum group metal particles having a particle size from 0.5 to 10 nm are disclosed, originating from precursor soluble salts, wherein at least 70% of the nanoscaled iron-platinum group metal particles are located on an outside surface layer of the metal oxide support material. This application further teaches that a uniform distribution of the PGMs located in the innermost layer of alumina is undesirable, since PGMs in the innermost layer of the alumina are not accessible for catalysis. There is a continuing need in the art to provide catalytic particles that provide excellent catalytic activity, thermal stability, and/or efficient use of a rhodium component.